俱舍論記 28.35

28.35

「即此四根本至起色想故」者，釋第三、第四句。因解無色，明除色想緣下地色，謂第四定色。餘文可知。無色雖亦緣下六地類智品道俱生戒色，今據有漏，言除色想。

As for "These four fundamental ones... because of giving rise to notions of form": this explains the third and fourth lines. In the course of explaining the formless, it clarifies the elimination of notions of form that take the form of the lower stage as object—namely, the form of the fourth dhyāna. The rest of the text is clear. Although the formless attainments also take as objects the morality-form arisen together with the path of the category of subsequent cognition (anvaya-jñāna) belonging to the lower six stages, here the elimination of notions of form is stated with reference to the sāsrava.